Modbus Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Simple and robust, it has since become a de facto standard communication protocol, and it is now a commonly available means of connecting industrial electronic devices. The main reasons for the use of Modbus in the industrial environment are:

developed with industrial applications in mind

openly published and royalty-free

easy to deploy and maintain

moves raw bits or words without placing many restrictions on vendors

Modbus allows for communication between many (approximately 240) devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.

The development and update of Modbus protocols has been managed by the Modbus Organization since April 2004, when Schneider Electric transferred rights to that organization, signaling a clear commitment to openness.

The Modbus Organization is an association formed of independent users and suppliers of Modbus compliant devices that seeks to drive the adoption of the Modbus communication protocol suite, and its evolution to address architectures for distributed automation systems across multiple market segments.

Cables

A MODBUS over Serial Line Cable must be shielded. At one end of each cable its shield must be connected to protective ground. If a connector is used at this end, the shell of the connector is connected to the shield of the cable. An RS485-MODBUS must use a balanced pair (for D0-D1) and a third wire (for the Common). “Connection of a crossed cable in a 2-wire MODBUS system may cause damages”.

Note : Category 5 Cables use other colors. For RS485-MODBUS, Wire Gauge must be chosen sufficiently wide to permit the maximum length (1000 m). AWG 24 is always sufficient for the MODBUS Data. Category 5 cables may operate for RS485-MODBUS, to a maximum length of 600m. For the balanced pairs used in an RS485-system, a Characteristic Impedance with a value higher than 100 Ohms may be referred, Especially for 19200 and higher baud rates.